Generally, a solid electrolytic capacitor is obtained by subjecting surface of an anode body of valve-action metal such as aluminium, tantalum, niobium, titanium or alloys of these metals to make it rough with micropores on the micron order formed thereon to thereby increase the surface area, forming a dielectric oxide film through chemical formation, impregnating a solid electrolyte via a separator between the film and the anode part or forming a solid electrolyte layer therebetween, forming a cathode electroconductive layer from carbon paste and a metal-containing electroconductive paste thereon, welding the body to a lead frame which serves as an external electrode and then forming an outer casing of epoxy resin or the like.
Particularly, since solid electrolytic capacitors using as solid electrolyte electroconductive polymers can have reduced equivalent series resistance and leakage current as compared with solid electrolytic capacitors using manganese dioxide as solid electrolyte, they are useful as capacitors meeting demands for higher performance and downsizing of electronic devices and many production methods have been proposed.
When a high-performance solid electrolytic capacitor is produced by using an electroconductive polymer, particularly in case of using a valve-action metal foil, it is indispensable to ensure electrical insulation between anode part serving as anode terminal and cathode part consisting of electroconductive layer containing electroconductive polymer. However, in a step of impregnating or forming solid electrolyte, it sometimes occurs that solid electrolyte intrudes into the anode region, so-called “crawling-up”. In such a case, insulation failure is caused between the anode part and the cathode part.
Examples of shielding measure for insulating the anode part of solid electrolytic capacitor from the cathode part include a method where after a polyamic acid film is formed by allowing a solution containing polyamic acid salt to electrodeposit on at least one part of valve-action metal having no solid electrolyte formed thereon, a polyimide film is formed by dehydration and curing with heat on the part (Japanese Patent Application Laid-Open No. H05-47611), a method where for the purpose of preventing crawling-up of solid electrolyte, a tape or a resin-coating film of polypropylene, polyester, silicone resin or fluorine resin is formed (Japanese Patent Application Laid-Open No. H05-166681) and a method including a coating step of a masking material solution which infiltrates into a dielectric film and forms a masking layer on the infiltrated part (International Publication No. WO00/67267 pamphlet (EP 1193727)
In the method of forming a polyimide film through electrodeposition (Japanese Patent Application Laid-Open No. H05-47611), it is advantageous as compared with general coating methods in that the film can be formed even on the fine pores, however, the method including electrodeposition step requires high cost and further, dehydration step at a high temperature is required to form a polyimide film. In the method of using a tape or coating film made of insulative resin for the purpose of preventing crawling-up of solid electrolyte in production process (Japanese Patent Application Laid-Open No. H05-166681), it is difficult to cover the edge parts of the substrate with the tape (film) completely, and sometimes solid electrolyte intrudes into the anode part. In production method of solid electrolyte including a coating step of a masking material solution which infiltrates into a dielectric film and forms a masking layer on the infiltrated part (International Publication No. WO00/67267 pamphlet), the coating solution cannot infiltrate deep enough when the surface condition of the dielectric film or formation condition of the fine pores such as pore size distribution is not suitable for the infiltration.
As described above, none of conventional masking methods is satisfactory. There have been demands for masking type (structure) which can ensure insulation between the anode part and the cathode part in a solid electrolytic capacitor.
Patent Document 1: Japanese Patent Application Laid-Open No. H05-47611
Patent Document 2: Japanese Patent Application Laid-Open No. H05-166681
Patent Document 3: International Publication No. WO00/67267 pamphlet